Abstract

Nitrogen fixation is an important process that converts atmospheric gaseous nitrogen, a form plants cannot utilize, into ammonia that can be easily assimilated. Large serine recombinase XisF (fdxN element site-specific recombinase), together with controlling factors XisH and XisI, plays a critical role in the expression of nitrogen fixation genes of certain Anabaena and Nostoc species of cyanobacteria. All three proteins are required to excise the fdxN DNA element from the chromosome in differentiating heterocysts for the expression of nitrogen fixation related genes. We report the first crystal structures of XisH and XisI proteins, both adopting novel protein folds. Based on the analysis of their sequences and structures, we propose that XisH and XisI proteins function as endonucleases and recombination directionality factors (RDFs), respectively.